Most consumer cameras are designed such that the lens and image sensors are parallel, and their centers fall on a line orthogonal to the image sensor. This arrangement generally produces good photographic results. However, due to perspective projection, parallel lines in the scene can appear to converge in the image. This phenomena is known as perspective distortion.
Perspective distortion is well known in photography. Traditionally, a view camera permits the photographer to avoid perspective distortion. This is accomplished by having independent control over the position of both the lens plane and the image plane. Lines in the scene that are parallel to the image sensor plane will appear parallel on the image. A view camera permits the photographer to control the character of the distortion that occurs by projecting a three-dimensional scene onto a two-dimensional image plane. The controls can be used to either reduce, modify, or increase the perspective distortion. View cameras are expensive and complex due to the many moving parts. In addition, traditional view cameras use film and lack many of the features of modern digital cameras.
Image warping is a well-known tool that may be used to modify the apparent perspective of an image. For example, U.S. Pat. No. 5,651,075 by Frazier et al., describes a method of compensating for the perspective distortion present in an image of a license plate captured at a known camera to subject distance. This patent describes a method of modifying the perspective by using the known relationship between the camera and the subject, which relationship is always constant. This arrangement would not be practical for general use at reducing perspective distortion.
In U.S. Pat. No. 5,227,889, Yoneyama et al describe a video camera having a sensor for detecting slant of the camera. The image is then rotated to remove the effect of the slant from the image. Their video camera cannot reduce perspective distortion.
In U.S. Pat. No. 5,900,909, Parulski et al describe a digital camera having a sensor for detection the orientation of the camera at the time an image is captured. Based on the orientation, the image is rotated by a multiple of 90 degrees so that the top of the image corresponds to the “up” direction. This camera cannot reduce perspective distortion in the image.
In U.S. Pat. No. 5,528,194, Ohtani et al describe a camera and processing to apply a geometric transform to an image captured with the camera. The geometric transform is derived based on a measured azimuth angle between the camera and the subject. According to col. 4. lines 10-14, the sensor measuring the azimuth needs to record the relative angle to the subject being photographed. The sensor can be a GPS (global positioning satellite) or a geomagnetic sensor that determines a three-dimensional position of both the camera and subject to determine the azimuth angle. GPS and geomagnetism measuring devices are expensive. In addition, camera requires determining the three-dimensional position of the subject, which is often difficult or impossible when the subject is inaccessible.